The present invention relates to photomultiplier tubes and particularly to an improved sensing means for monitoring the current due to photoelectrons emitted from a photocathode during the formation thereof.
In the prior art it has been found that bialkali and multialkali photocathodes with improved photocathode sensitivity may be made by carefully monitoring the photocurrent during each step of the photocathode formation process (see for example U.S. Pat. Nos. 3,838,304 issued Sept. 24, 1974 to A. F. McDonie for "Method of Making A Bialkali Photocathode With Improved Sensitivity and High Temperature Operating Characteristics," and 3,658,400 issued Apr. 25, 1972 to F. A. Helvy for "Method of Making A Multialkali Photocathode With Improved Sensitivity To Infrared Light And A Photocathode Made Thereby.") A typical photocurrent monitoring curcuit is shown in U.S. Pat. No. 3,434,876 issued Mar. 25, 1969 to R. G. Stoudenheimer et al for "Photosensitive Cathode." The monitoring circuit is reproduced as prior art FIG. 1.
As shown in FIG. 1, photoemission from a photocathode 10 within an envelope 12 is accomplished by locating a light source 14 adjacent to the photocathode 10.
The photocathode 10 is made negative with respect to a pair of leads 16 by means of a potential source 18. A microammeter 20 is connected in series in the circuit of the potential source 18 to measure any electron discharge between the photocathode 10 and the positive leads 16 which are connected to a primary electrode 22 which serves as an anode or sensing electrode. The sensitivity of the photocathode is expressed in terms of microamperes of emitted electron current per lumen of light incident on the photocathode 10.
In the prior art it has been noted that a substantial electrical leakage or "dark" current occurs during formation of the photocathode. The "dark" current is believed to be caused to some extend by thermionic emission of electrons from the photocathode but predominantly by a current of ionized alkali gases which form conductive paths in the region between the photocathode on the wall of the envelope and the sensing electrode. The dark current is undesirable because it is sometimes mistaken as a true signal. It also persists during the presence of a light input to the photomultiplier tube, thereby creating a spurious signal in the presence of the true signal such that the output of the tube is obscured because of a poor signal-to-noise ratio. The problem has been sufficiently acute, especially during the initial alkali evaporation steps when the photocathode sensitivity is low, that the resulting photomultiplier tubes do not have the desired optimum sensitivity or reproducibility of sensitivity from one tube to the next that is required.